This invention relates to electronics, and more particularly to compositions useful for producing conductor patterns adherent to substrates.
Conductor compositions which are applied to and fired on dielectric substrates (glass, glass-ceramic, and ceramic) usually comprise finely divided inorganic powders (e.g., metal particles and binder particles) and are commonly applied to substrates using so-called "thick film" techniques, as a dispersion of these inorganic powders in an inert liquid medium or vehicle. Upon firing or sintering of the film, the metallic component of the composition provides the functional (conductive) utility, while the inorganic binder (e.g., glass, crystalline oxides such as Bi.sub.2 O.sub.3, etc.) bonds the metal particles to one another and to the substrate. Thick film techniques are contrasted with thin film techniques which involve deposition of particles by evaporation or sputtering. Thick film techniques are discussed in "Handbook of Materials and Processes for Electronics", C. A. Harper, Editor, McGraw-Hill, N.Y., 1970, Chapter 12.
The most commonly used conductor compositions employ noble metals, especially gold, silver, platinum, palladium, and their mixtures and alloys, despite their relative costliness, since their relatively inert characteristics permit firing in air.
The prior art on conductor compositions includes the use of glass binders for noble metals, as well as the use of glass-free binders. Martin, U.S. Pat. No. 3,293,501, discloses conductive films of glasses comprising copper and bismuth oxides. Buck, et al., U.S. Pat. No. 3,776,769, discloses compositions of noble metal, copper oxide and glass, fireable in nonreducing atmospheres. Short, U.S. Pat. No. 2,819,170, discloses compositions of silver and a vitrifiable flux of bismuth oxide and cadmium borate.
German patent publication OS No. 2,424,579 (published Oct. 16, 1975; assigned to Jenaer Glaswerk Schott) discloses sintered composites of glass and copper and/or copper alloy made by molding or extrusion of powder starting materials or as conductive coatings. Specifically, claimed are compositions of copper or copper alloy, 0.8-8% copper oxide and 0.8-10% oxides of group IV-VI elements. It is preferred that the copper oxide be part of the glass powder, but also disclosed are physical mixtures of metal, glass, copper oxide and group IV-VI oxide (col. 4, lines 57-59). The only proportions disclosed are in the examples and are 1 part glass powder to 3 parts copper powder (col. 7, line 14). The powders are heated in a reducing atmosphere. No suggestion is made to fire these materials in a neutral atmosphere.
U.S. Pat. No. 4,072,771, issued on Feb. 7, 1978 to J. D. Grier, discloses copper thick film conductor pastes which comprise a solids mixture containing 92-97% by weight of copper, 1-5% by weight of copper oxide, and 1-10% by weight of glass frit. The solids mixture is made into a paste by mixing it with a fluid vehicle. The thick film is fireable in nitrogen atmosphere. It is required that there be an intentional and controlled preoxidation of the copper to form controlled levels of surface oxidized copper powder to achieve a satisfactory conductor. Lead borosilicate glass, with a lead oxide content of about 65% lead oxide, is the preferred glass frit disclosed.
The industry needs a less expensive conductor composition, based on a base metal such as copper, which can be fired in a neutral atmosphere (such as nitrogen), rather than the more expensive (and dangerous) hydrogen atmospheres sometimes used. The compositions should be capable of producing fired (sintered) films of good adhesion to typical substrates such as alumina, and have adequate solderability, or both. Also, good wetting of the copper by the glass employed is a must.